<!DOCTYPE html>
<html lang="zh-CN">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 50vw;
      height: 50vw;
      margin: 0 auto;

      /*
        box-shadow: [inset] offset-x offset-y blur-radius spread-radius color;
         ● inset          指定 inset 会使阴影向内扩散
         ● offset-x       阴影偏移量
         ● offset-y
         ● blur-radius    阴影模糊半径
         ● spread-radius  阴影扩散半径
         ● color          阴影颜色

        可以设置多个阴影，用 , 分隔
      */
      box-shadow: inset 0 0 3px 0 #17f, 0 0 3px 0 black;
    }
  </style>
</head>

<body>
  <div></div>
</body>

</html>